If you don’t know why you need a Machinery’s Handbook, you don’t need one!
In the workshop I rely on my head and a few laminated sheets. Beyond that Tubal Cain’s “Model Engineer’s Handbook” has most of what I need, supplemented by the web.
My 1949 Newnes Engineer’s Reference Book covers the bonkers world of obsolete Imperial Measure very well, and a few older books explain Admiralty and other long gone thread systems. Sadly I don’t have a good reference on pre-WW2 metric as practised somewhat differently by Belgium, France, Germany, Italy, Japan, Russia, Sweden, Switzerland and others.
My 21st Edition Machinery’s is good for US, British, and post WW2 metric, and it’s almost comprehensive. The downside is it’s massive, most of the information is irrelevant to me, and the print is tiny! I only refer to Machinery’s when all else has failed, and then it’s wonderful.
Dave